1. Field of the Invention
The present invention relates to an imaging and flow measuring device for evaluating patency and blood flow characteristics in coronary bypass grafts.
2. Prior Art
More than 70,000 coronary bypass graft operations are performed each year in the United States. Premature closure of these grafts is not rare, especially during the first post-operative year and sometimes occurs within a few hours of the bypass operation. When a bypass patient presents with chest pains that could indicate a blocked graft, the decision to reoperate on that patient is based upon an evaluation of blood flow through the patient's graft or grafts. The most common method for this evaluation is direct cineangiography. With this technique a catheter is passed via an artery to the ostium of the bypass graft. The tip of the catheter is inserted into the graft and a radiographic contrast agent is injected while an x-ray fluoroscopic image of the heart and graft vessel is being recorded. The patency of the graft is determined by whether or not the contrast agent is seen passing through the graft in the contrast enhanced image. This method, although effective requires hospitalization, minor surgery to gain access to an artery, exposes the patient to a substantial radiation dose, and because it transiently occludes the graft during injection of the contrast agent, carries some degree of morbidity and mortality. Although a number of safer alternative methods have been tried for evaluating the patency of bypass grafts, only two, cine-computed-tomography and magnetic resonance imaging (MRI) have achieved sufficient success to warrant serious consideration as alternatives to cineangiography.
With cine-computed-tomography, parallel tomographic image slices (usually 2 to 8 images) of the chest cavity are obtained simultaneously. Eight to ten sets of such images are obtained at approximately 50 millisecond intervals. With proper timing of these images, the relative timing of the passage of an intravenously injected radiographic contrast agent passing sequentially through the image slices can be obtained. Bypass graft patency and flow characteristics are determined from the appearance and timing of the contrast enhanced bright spots in the images.
With magnetic resonance imaging, two methods of graft evaluation have been employed. With spin-echo-MRI a single planar slice image through the chest cavity is obtained. With this technique a patent bypass graft will appear as a negative signal (i.e. a small dark spot in the image). With the alternative cine-MRI technique flow through a bypass graft will appear instead as a bright signal. Although the advantages of MRI techniques (x-ray exposures and radiographic contrast injections are not required) are significant, the MRI techniques do not achieve the accuracy of either cine-computed-tomography or cineangiography. Some of the difficulties with the MRI techniques include blood flow in normal vascular structures, hemostatic clips, mediastinal fibrosis or pericardial fluid, all of which can mimic the signal produced by a patent graft.
The cine-computed-tomography technique also has its limitations. Difficulties with graft patency and flow evaluations with this technique include false negatives (bright spots) due to surgical clips or calcified blood vessels or false positives (absence of bright spots) due to unusual bypass graft positions. Graft identification with cine-computed-tomography scans can also be made difficult by patient-to-patient variations in the timing of the arrival of the injected radiographic contrast agent. These scans are also expensive and require sufficient radiation exposure to preclude long term follow-up studies on individual patients. In addition, cine-computed-tomography requires, as does cineangiography, surgical access to a patient's arterial blood supply.
In spite of the significant progress with the MRI and cine-computed-tomography techniques, their limitations are such that hospitalization for cineangiography is still the method of choice in the vast majority of bypass graft evaluation studies. Most of the limitations with the MRI and cine-computed-tomography images relate to the association of graft identification with graft evaluation. If an isolated bright spot appears in the image it can be a patent graft, an artifact, or other structure. On the other hand if an isolated bright spot does not appear the bypass graft is not patent, not in the image plane, or hidden.
The present invention avoids many of the problems associated with other attempts to provide a safe effective evaluation of bypass graft patency. With this invention graft identification and graft evaluation are separate procedures. The location of a bypass graft within the chest cavity is determined, prior to a radiographic contrast injection, with a precise x-ray scatter profiling technique. Problems associated with timing the arrival of the radiographic contrast agent are avoided by continuously monitoring the bypass graft until the contrast agent makes its first pass through the heart. Continuous monitoring with the cine-computed-tomography or cineangiography techniques is not feasible because of substantial radiation exposure. Hardware for the present invention is less expensive than that required for the alternative techniques and the radiation dose per bypass graft evaluation is only 1/10 to 1/50 of that of cine-computed-tomography or cineangiography. The low radiation exposure of this invention makes it quite suitable for use as a screening device or for long-term follow-up studies of patient bypass graft patency. In addition, the application of the present invention eliminates the need for surgical access to the patient's arterial blood supply.